200 asymptomatic blood donors identified as anti-HTLV-III positive during routine donor screening by the Washington Red Cross and Department of Transfusion Medicine, NIH will be enrolled in a long-term prospective follow-up. In addition, there will be 100 controls consisting of blood donors who have false positive tests for anti-HTLV-III (ELISA positive, but not confirmed by Western blot). Each group of donors will: 1) be evaluated for clinical evidence of AIDS or AIDS-related disorders; 2) have extensive evaluation for AIDS-related risk factors; 3) have the specificity of their anti-HTLV-III antibody determined by confirmatory tests and Western blot analysis; 4) be cultured for the HTLV-III virus; 5) have an extensive immunologic evaluation for AIDS-related immune defects; 6) be tested for HTLV-III antigens. Donors will be followed and retested every 6 months for at least 5 years. Donors who develop evidence of AIDS will be offered evaluation and possible treatment in other NIH AIDS protocols.